externalfield

External field, in physics and related disciplines, refers to a field that is imposed on a system from outside and treated as non-dynamical within the model. It is distinguished from fields generated by the system’s own sources, which are considered internal or self-consistent. External fields are used to probe, control, or define the behavior of a system without incorporating the back-reaction of the field on its sources.

In classical contexts, external fields include gravitational, electric, magnetic, and optical fields that influence particle motion or material properties. For example, an external magnetic field can alter the trajectory of a charged particle, while an external electric field can polarize a dielectric. In many problems, the external field is specified by boundary conditions or by a prescribed value, rather than by solving a coupled set of dynamical equations for both field and matter.

In quantum mechanics and quantum field theory, external fields are often treated as classical background fields or sources. This approach simplifies calculations by keeping the field non-quantized while studying how quantum systems respond to it. The Furry picture and the background field method are well-known formalisms that incorporate strong external fields into relativistic and gauge-theoretic analyses.

Applications span particle physics, condensed matter, and astrophysics, including strong-field QED, magneto-optical experiments, and the modeling of fields produced by massive astronomical bodies. External fields are essential for understanding how systems behave under imposed conditions, independent of the fields’ own dynamics.

See also: background field, external potential, Lorentz force, Maxwell's equations with sources, background field method.